Normally, a roller used to convey sheets is supported at both ends of its rotational axis. When a load acts on the roller, the roller bows in its axial center region. Consequently, compressive force applied by such rollers to a sheet and forces such as pressure between the contacting surfaces of the sheet and rollers and pressure between the contacting surfaces of the rollers (hereinafter collectively referred to as “the nipping force”) are smaller in the axial center region of the rollers.
Since slippage is more likely to occur between the sheet and rollers when the nipping force is smaller, a reduced nipping force in the axial center region of the rollers may lead to sheet-conveying problems. To address this issue, the invention disclosed in Japanese unexamined patent application publication No. HEI-8-91635, for example, attempts to avoid a reduced nipping force in the center region of a pair of rollers disposed in confrontation with each other by applying pressure to the axial center region of at least one roller in a direction toward the other roller.